The invention relates in general to gap-solitons, and in particular to the formation of gap-solitons using photonic crystal fibers.
The amount of digital data transported around the globe grows rapidly every year. Whenever large amount of data has to be transferred, or whenever the data needs to be transferred across large distances, optical fibers are the preferred medium for transportation, due to their low losses, and high capacity. A single optical fiber (whose core is less than 10 μm in diameter) has been demonstrated to transmit more than 1 Tbit/sec, with losses less than 0.2 dB/km.
By the same token, this large amount of data needs to be processed. Currently, the capacity of an optical fiber is split between many channels, each of them carrying 10 Gbit/sec or less. Since each of the channels needs a separate set of components to operate, the tendency of the industry has been to aim at higher and higher bit rates, thus reducing the number of components in the system. Bit rates higher than 10 Gbit/sec have been demonstrated experimentally but are not yet used commercially. The reason for this is that reducing the number of components is beneficial only if the higher bit rate components are of sufficiently lower price. For example, using 40 Gbit/sec components instead of 10 Gbit/sec components is beneficial only if the price of the 40 Gbit/sec components is less than 4 times as high as the price of those operating at 10 Gbit/sec. Unfortunately, 40 Gbit/sec components are not yet commercially competitive.
To perform almost any kind of operation on optical signals today (wavelength conversion, pulse regeneration, bit-rate conversion, logic operation, etc.) the signals first need to be converted to the electronic domain. Unfortunately, there are fundamental physical reasons that prevent electronics from operating well at high frequencies. As a result, the price of electronic components grows rapidly when higher bit-rates are needed. At 40 Gbit/sec (which corresponds to operating frequency of roughly 100 GHz) there are no commercially satisfactory products today. Consequently, using all-optical signal processing becomes rapidly more and more appealing.
Since the signal processing needs to be done on ultra-fast scales, the only mechanism at disposal is to exploit a material's optical non-linearities. Most of the research in the area of all-optical devices today is in high-index-contrast integrated optics. Such devices suffer from large losses due to roughness at the surfaces of their waveguides, are highly polarization sensitive, and it is extremely difficult to couple light into them. Furthermore, since they are built using lithography, production typically involves large and very expensive semiconductor fabrication facilities.
Using in-fiber all-optical devices would solve all the problems mentioned in the previous paragraph. Silica fiber non-linearities are very low since the Kerr coefficient is very small (2.6×10−20 m2/W), and since the modal area is large (100–10 μm2). As a result, a typical telecomm signal (5 mW peak power per channel, immediately after amplifiers) needs to be propagated for very long distances before the non-linear effects become noticeable. Recently, a new type of fiber (called OmniGuide fiber) has been proposed. Materials used in this fiber have Kerr coefficients 100–1000 times higher than in silica. Furthermore, since this is a high-index contrast fiber, modal areas can be 1000–100 times smaller. Consequently, the non-linear response of these fibers is 4–6 orders of magnitude better than in silica fibers, making them an ideal non-linear medium.
Previous experimental work on gap-soliton properties and all-optical switching operation include Fiber Bragg gratings. They utilize a weak index grating written in the core of an optical fiber. It can be used for almost all nonlinear phenomena and devices. However, because of the weak grating (necessary in order to minimize scattering losses) the spectral content of the gap is very small. Combining this with the very small nonlinear coefficients of silica, enforces the need of many grating periods for the nonlinear effect to build-up and induce switching. Typical fiber lengths in such experiments are 1–10 cm, and require light powers of 10–30 GW/cm2.
The use of integrated multilayer heterostructures was integral in earlier work of gap-solitons. Typically, the multilayer heterostructures utilize an AlGaAs Bragg grating heterostructure. They all generally have a small spectral gap (<1 nm), but greatly vary in the reported device size (ranged from 5 μm to 5 mm) and in operation power (1 kW/cm2 to 1 GW/cm2). Basic drawback is that production typically involves large and very expensive semiconductor fabrication facilities.
In both Bragg fibers and multilayer heterostructures, the grating provides the necessary spectral gap (i.e. frequency regions where waves cannot propagate) in order to observe gap-soliton bistability. However, it is the grating as well that introduces the restrictions of small spectral gap, long device sizes, expensive fabrication facilities etc. In order to circumvent the grating, we have to find other ways to induce a spectral gap for propagation.
There are many ways to induce such a change to the propagation properties of a photonic crystal fiber, and if done properly, they should all produce the desired effect. One efficient way would be by introducing a constriction on part of the fiber's length. Constricted photonic-crystal-fiber devices have been demonstrated with the Holey fiber. It was used to just locally reduce the modal area and thus enhance the nonlinear effect. In other cases it was engineered so that the modal area extended inside the cladding where it interacted with suitably placed micro-fluids inside the cladding holes.
Many device operations were demonstrated, even generation of soliton trains. Such soliton trains can easily appear in Kerr media, but are not gap-solitons, which are essential for the bistable operation. If the constrictions are chosen carefully having in mind the creation of a spectral gap for propagating guided modes, then gap-solitons are possible.